Controlling photophysics and photochemistry via quantum superpositions of electronic states: towards attochemistry

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When molecules absorb light of sufficient energy, an excited electronic state is generated. The distribution of electrons - the chemical bonding - then changes, causing the nuclei to move in response. Electronic changes such as these appear to be instantaneous relative to the nuclear motion that follows. We now know that these electronic changes aren't instantaneous, but they are usually much faster than the nuclear motions, which has limited our scope for controlling their effects until now.We propose to explore how laser manipulation of electronic state motion can offer unprecedented control over photochemistry: chemical reactions that are initiated by changes in bonding in electronic excited states. Can we direct the outcome of a photochemical process in a molecule by controlling the initial evolution of a coherent quantum superposition of its electronic states? Our proposed research will explore a new approach to controlling dynamics in molecular systems at an important interdisciplinary junction. It promises to benefit our understanding of - and mastery over - ultrafast chemical processes, and to extend our ability to manipulate quantum states of matter into the attosecond time domain.Recently attosecond molecular physics has been exploring the concept of "charge migration": electronic dynamics following sudden excitation of an electron in a molecule or other extended quantum system. To understand such phenomena we must recognise the quantum nature of both electrons and nuclei. Ultrafast decoherence due to coupling of the evolving electronic and nuclear quantum states is found to be rapid and general, taking place on a timescale of a few tens of femtoseconds. The control of photoexcited quantum state dynamics can therefore only be achieved with light fields applied on a faster timescale, before decoherence removes our scope for control. A central target of this research is the control of quantum evolution by ultrafast light fields in the vicinity of conical intersections: molecular geometries where crossings between electronic states can lead to multiple chemical pathways. Control here will give us control over different chemical outcomes. Excitation-control-probe sequences of light pulses will be applied to selected molecules. A few-femtosecond UV excitation pulse will initiate the electronic state superposition, followed by a few-cycle infrared pulse after a short and precisely controlled time delay. This pulse sequence will manipulate the coherence between states as the system flows through the critical conical intersection. By varying the superposition within a time interval of a few tens of femtoseconds in this way - on a time-scale faster than decoherence - we will change the quantum evolution path and final outcomes. This path will be measured in real-time via X-ray spectroscopy with sub-femtosecond X-ray pulses, a technique with a high sensitivity to molecular structure and electronic states. Computer simulations using state-of-the-art codes and substantial computing power to solve the coupled electronic-nuclear motions will be used both to predict and to explain the experiments.We will study the dynamics and control of small, isolated, molecular systems in this proposal. Nevertheless, it is likely that what we will learn through this research will be applicable to the quantum scale manipulation of many other light-absorbing systems with ultrafast chemical dynamics. This work is therefore pertinent to a wide range of nanoscale systems: nanoparticles, catalytic complexes, biomolecules, organic optoelectronics, two-dimensional materials and other advanced materials. As well as providing new insight into the fundamental behaviour of molecules, the ultrafast quantum science we are researching may lead to future quantum devices where the flow of charge, energy and information within a quantum system can be controlled by ultrafast light fields.